This invention relates generally to a method and apparatus for media blasting and more specifically relates to an improved method and apparatus for media blasting a workpiece or part. This invention preferably uses the powered part hold-down apparatus of U.S. Pat. No. 5,272,897, the disclosure of which is hereby incorporated by reference.
In the finishing of machined or fabricated parts or workpieces, the use of media blasting or peening to increase the fatigue strength of a workpiece or part is well known. An example of such a part is a gear utilized in automobile transmissions. Once these gears are formed, they should be processed by media blasting machines and the like to ensure that the gears meet government and manufacturer specifications, as well as to ensure that the gears are suitable for performing their intended functions.
Usually, the workpiece or part is placed in a closed chamber, in which a blasting means or a blasting system is located. A typical blasting system is a suction type system, which includes a media feed hose and an air supply hose interconnected at a suction gun. When the blasting system is actuated, the suction gun is turned on. This, in turn, causes media to be drawn from a storage hopper and into the media feed hose. The media are drawn through the media feed hose to the suction gun, where the media are then mixed with air from the air supply hose. After mixing of the media and air, the suction gun directs a stream of the air/media mixture against the workpiece, to provide compression stress to the surface by means of the colliding media.
Media blasting operations/systems of this type have a number of problems. Although media blasting strengthens the workpiece, it may also roughen the surface of the workpiece and/or result in an uneven surface finish. Surface roughness or an uneven surface finish decreases the hardness of the workpiece and can interfere with its intended function. For example, surface roughness or uneven surface finish in a gear interferes with the ability of the gear to mesh. Moreover, a workpiece having an uneven surface finish is susceptible to crack propagation, which can result in workpiece failure. Thus, it is important that the surface finish be evenly dimpled with a uniform distribution of peaks and valleys, as an uneven distribution contains deeper valleys which can propagate as cracks.
A further problem with media blasting operations/systems of this type result from the use of suction means to provide the media blast stream. As discussed above, in media blasting or shot peening operations, the dimple formation on the workpiece must be carefully controlled. In regulating dimple formation, the velocity of the media is an important control parameter. However, with suction guns it is difficult to control the flowrate of the media blast stream. It is also difficult to adjust the flowrate when changing from use of one type of media to another type, as the air velocity and media flow rate cannot be adjusted independently. Moreover, in a suction gun the media and air are mixed immediately prior to ejection, which may not allow the media to develop sufficient velocity to provide desired compressive stress.
Prior art methods have attempted to solve the problems of surface roughness or uneven surface finish by using glass beads. Although glass beads tend to reduce surface roughness, they may not sufficiently increase the fatigue strength of the workpiece. A further problem with this and other prior art methods is the difficulty of obtaining even peening of the workpiece surface. This problem arises, in part, from the lack of directivity of the media. Prior art methods also suffer from a high wastage of media, due to attrition of the media during the blasting process. The high wastage of media reduces both the operational efficiency and cost-effectiveness of a blasting operation.
Japanese patent application number H6-217214, entitled xe2x80x9cA Strengthening Method For Metal Parts,xe2x80x9d filed Sep. 12, 1994, in the name of Honda Motor Company, Ltd., describes a media blasting method which attempts to alleviate some of the above mentioned problems. This Japanese application describes a metal strengthening method in which a stream of water and glass beads is directed toward a metal part. Use of water as a suspending medium imparts directivity to the glass beads. It also provides additional compression stress and provides a polishing effect. Although use of water improves the surface smoothness of the workpiece and the directivity of the media, it introduces additional problems without addressing the problem of media wastage or media flow control. In particular, use of wet media complicates design of the media blasting system, because all the parts of the media blasting system have to be designed for the presence of water. For example, care must be taken to avoid creating an electrolytic junction. Use of water also limits the type of workpieces that can be processed and the type of media which can be blasted. For example, it would be undesirable to use this method with a workpiece which is susceptible to corrosion.
Accordingly, a media blasting apparatus and method according to the teachings of the present invention is intended to solve problems inherent in the prior art media blasting systems discussed above, as well as others. A media blasting system and method according to the present invention increases operational efficiency, reduces media wastage and media use, provides even peening of a workpiece surface, provides a polishing effect or an even surface finish, and, moreover, obviates use of a high directivity media stream.
Another approach to providing gears with the desired surface wear and strength characteristics has been to coat the finished gears with various surface treatments such as Diamond Black and Balzers. One such treatment is sold by Balzers WC/C under the tradename Balimit WC/C(trademark). Another such surface treatment is sold under the tradename Black Diamond(trademark) by Body Coat. Such surface treatments can be disadvantageous for two reasons. First, the gears must be coated after they have been manufactured which typically requires the machined gears to be shipped to a specialty coating vendor to undergo a time-consuming, multi-step process and then be returned to the manufacturer. This process of shipping and coating off-site can slow the manufacturer""s production cycle. Secondly, the use of such surface treatments is frequently far more expensive than is desired.
It has also been recently discovered that for certain metal surfaces, media blasting with glass or steel media has been less effective than is desirable for workpieces, such as gears. For instance, it has been observed that the use of glass or steel media for blasting of gear surfaces made from such metals can result in ill-defined dimples which are believed to provide less retention of oil on the gear surface. This is believed to result in less lubrication on the gear surface during operation in a transmission which can contribute to faster wear and more gear tooth pitting than is desirable.
Another problem with prior conventionally peened gears is the formation of xe2x80x9cplus tipsxe2x80x9d. Plus tips are formed when the tip of the gear tooth peak becomes flattened and slightly undercut on the adjacent area of the gear face during peening. The formation of xe2x80x9cplus tipsxe2x80x9d on gear tooth peaks are associated with excessive gear noise in the transmission during operation.
A general object of the present invention is to provide a media blasting apparatus and method which provides improved workpiece peening.
Another object of the present invention is to provide a media blasting apparatus and method which reduces media use and wastage.
An additional object of the present invention is to provide a media blasting apparatus and method which obviates use of a high directivity media stream.
A further object of the present invention is to provide a media blasting apparatus and method which allows control of media flowrate by using a pressurized system.
Yet another object of the present invention is to provide a media blasting apparatus and method which provides even peening of a workpiece surface and a uniform surface finish.
An additional object of the present invention is to provide a media blasting apparatus and method which provides effective peening of hard-to-reach surfaces of a workpiece.
A still further object of the invention is to provide a method for processing gears to provide improved gear root radius strength, reduced gear tooth surface pitting, avoidance of xe2x80x9cplus tipxe2x80x9d formation on gear teeth, as well as more and better defined surface dimples on the gear.
Further objects and advantages of the present invention will become apparent from a consideration of the drawings and ensuing description.
Accordingly, one embodiment of the present invention provides a media blasting apparatus in which a workpiece to be treated is held in a predetermined position during processing by the powered part hold-down apparatus of U.S. Pat. No. 5,272,897 and including a media reclaim and recycle system. The present invention also provides a media blasting method in which a smaller mass flow rate of media is blasted at higher velocity and for a longer time than in prior art methods to produce desired results.
The present invention further provides a method of media blasting a workpiece with a ceramic blasting media including the steps of positioning a workpiece in a part holder for maintaining the workpiece in at least one predetermined position during a blasting operation, supplying positive air pressure to deliver ceramic media to a blast nozzle, directing a ceramic media mix at the workpiece through the blast nozzle, exposing a plurality of surfaces on the workpiece to the ceramic media and collecting the ceramic media for reuse in a subsequent blasting operation.
The present invention further provides a method of media blasting a metallic gear with a ceramic blasting media including the steps of positioning a workpiece in a part holder for maintaining the workpiece in at least one predetermined position during a blasting operation, directing a ceramic media mix at the workpiece, and exposing a plurality of surfaces on the workpiece to the ceramic media.
The present invention further provides a method of media blasting a metallic gear with a fine steel media including the steps of positioning the metallic gear in a part holder to maintain the metallic gear in one or more predetermined positions during a blasting operation; directing a metallic shot having a diameter of between about 150 microns and about 200 microns at the metallic gear part; and exposing a plurality of surfaces on the metallic gear to the directed media.